Our general aims are (1) to determine the relation between the light stimulus incident on a human eye and the visual signals generated by the rod and cone receptor which absorb the light; and (2) to elucidate the neural mechanisms that modify these signals as they are transmitted through the retina. Current investigations are concentrated on determining the loci of sensitivity regulation during dark adaptation, and on clarifying the mechanism of rod saturation. BIBLIOGRAPHIC REFERENCES: Conner, J.D., & MacLeod, D.I.A. Rod photoreceptors detect rapid flicker. Science, 1977, in press. Boynton, R.M., Hayhoe, M.M., & MacLeod, D.I.A. The gap effect: visual discrimination as affected by field separation. Optica Acta, 1977, in press.